Orchestration App Test Restructure

Goal

Merge apps/orchestration/tests.py (22 tests, 6 classes) into the existing _tests/ package per agents.md.

Mapping

Source class	Target file
DTOSerializationTests	_tests/test_dtos.py (new)
PipelineRunModelTests + StageExecutionModelTests	_tests/test_models.py (new)
SignalTagsTests	_tests/test_signals.py (new)
OrchestratorTests + StageExecutionErrorTests	_tests/test_orchestrator.py (new)

Notes

• Zero overlap with existing _tests/ files
• No test logic changes — all 79 tests (57 existing + 22 moved) pass
• tests.py deleted after migration